hile concrete is a popular building material, known for both its adaptability and strength, it is also subject to deterioration and as such often needs repair.

“Concrete is only susceptible when it’s reinforced concrete,” says John O’Connell, from specialist remedial contractor Preservation Technologies and secretary of the Australian Concrete Repair Association (ACRA). “It’s the reinforcement that is needed as part of the design, but this corrodes. Most reinforcing steel can expand up to eight times its original size through corrosion. As the re-inforcement expands within the concrete, it puts stresses and strains on it, cracks it and the concrete spalls. That allows more atmospheric corrosion to take place and then you get a wholesale concrete deterioration.”

Steel corrodes for two reasons, O’Connell continues: “It’s either due to attack from chloride ions which come from the structure being near the sea; and through an atmospheric attack which is called carbonation that is due to too much carbon dioxide. The carbonation lowers the pH of the concrete to below 10 and that then creates a situation whereby the reinforcing steel is no longer protected by the alkalinity of the concrete, so it rusts, corrodes, expands and pops the concrete.”

Concrete deterioration can also result in water leakages into buildings as well as other health and safety aspects. “ACRA tried to highlight the dangers of balconies on residential buildings where deterioration was left unchecked and in a couple of instances balconies collapsed,” O’Connell says. “The other area would be bits of concrete falling off inner-city buildings onto pedestrians or traffic. Also if you had deterioration on the actual structure it could collapse, such as a bridge. This is a very worst-case scenario — you generally hear about structures having to have their load reduced.”

Concrete deterioration can also be a man-made problem, according to Hamish Anderson, manager of Remedial Building Services. “It could be the way the reinforcement beams were put in in the first place, whether they’d been treated for rust before placed or if there’s a small hairline crack in the concrete just from the pouring — if for example the temperature wasn’t right at the time and moisture can get into it, then the building settles and gets cracks which are not to do with reinforcement,” he explains.

It is important to treat the underlying problems of concrete spalling as well as the visible signs, Anderson says. “We understand there’s more problems underneath to do with the reinforcement, so we’ll go in and treat that underlying problem, then fix the concrete. We aim for a solution which lasts 10 or 15 years.”

The cost of repairing concrete can be very high. “The big cost in our industry is access cost; whether it be scaffolding or mass climbers,” O’Connell says. Anderson agrees. “Deferred maintenance as we call it ends up costing you more in the future; if you’re just treating the visual aspects, the problem will get worse and it’ll cost you more to fix it.”

To avoid this, prevention measures are essential. “Make sure the cover to the reinforced steel is installed,” O’Connell advises. “What happens, especially in older buildings, is you might only get a 10mm cover from the outside of the concrete to the first bar of steel and the design might have been 25-50mm. The steel beam being closer to the surface allows it to be corroded quicker. Also buildings should be coated with a protective coating that will stop the ingress of these atmospheric gases and chlorides into structures.”

But not everyone is heeding such advice.

While some builders continue to cut corners, remedial specialists will be in high demand, along with concrete repair products such as Sika’s systems. According to Sam Toglaw, technical representative — construction at Sika Australia, the company’s top products for remedial work are the Sika Monotop and Sika Gard systems. “The Monotop is the repair material — it has its own priming component and is suitable for overhead repairs, façade, vertical, horizontal, and floor slabs, with provisions for corrosion inhibitors and water tightness,” Toglaw says. “It’s applied by hand or even by spray application. Sika Gard is the final protection for remedial work, especially for façade restoration. That locks in your repair work and gives you something like a 10-year warranty which can be renewed for another 10 years upon inspection.”

According to O’Connell, regular five-10 yearly inspections by remedial engineers will prevent long-term damage and escalating costs.

Case study 1: Beachside apartments on Avoca Beach

Salt ingress to external concrete and masonry elements caused corrosion of structural steel in this apartment block. The original masonry balustrades obstructed ocean views and their removal offered the opportunity to raise the overall look of the building and boost property values.

The existing masonry balustrades, tiles and screed beds were removed. Deteriorated slab edges were repaired and a new reinforced concrete hob was constructed around each balcony to take the new stainless steel and glass balustrade system. The structural steel columns at each balcony were exposed, prepared and protected. Brickwork was then re-instated and new aluminium column cover plates fixed over the columns to provide further protection.

Case study 2: NSW Department of Education building

Due to ongoing water penetration through the concrete slab roof of the Department of Education offices in NSW. In a heritage-listed building in the Sydney CBD, Remedial Building Services was contracted. During the tendering process it was determined that the 1,500sm² roof was protected by a combination of Nuralite (a sheet membrane containing bonded asbestos) and a rubber sheet membrane over the top. This system was failing in a number of areas and had caused cracks in the concrete slab to be exploited by water ingression.

The process of removing the existing waterproof membrane was made more complex due to the existence of the Nuralite. Safe working procedures had to be followed to ensure the safety of both the contracting staff and the tenants in the building.

All drummy (hollow) concrete on the slab was removed and falls corrected to ensure correct drainage. A torch-on membrane was then applied to the roof.

The remedial works to the parapet walls were achieved using a protective membrane to prevent the ingress of water. Pressure strips were also used in several areas to completely seal the membrane.

Source: BPN

Architecture & Design Team

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